procedure TForm1.Button1Click(Sender: TObject);//На русский
   var
    Layout: array[0.. KL_NAMELENGTH] of char;
   begin 
    LoadKeyboardLayout( StrCopy(Layout,'00000419'),KLF_ACTIVATE);
   end; 

   procedure TForm1.Button2Click(Sender: TObject);//На английский
   var
    Layout: array[0.. KL_NAMELENGTH] of char;
   begin
    LoadKeyboardLayout(StrCopy(Layout,'00000409'),KLF_ACTIVATE);
   end;
	             		

Источник: www.sources.ru